1. Field of the Invention
The invention relates to data transmission apparatus and more specifically to means for automatically recognizing one of a plurality of calling modem types and for switching a compatible modem into the answering circuits.
2. Description of the Prior Art
Existing computer ports are capable of handling speeds of up to 2400 bits per second in the asynchronous mode and of speeds of 2400 bits, 4800 bits, 9600 bits, and 19.2 kilobits per second in the synchronous mode. However, existing modems that connect to the computer port are restricted in speed. Six examples of such modems are the Bell 103 operating at 300 bits per second; the Bell 202, operating at 1200 bits per second; the Bell 201 operating at 2400 bits per second; the Vadic 3400 operating at 1200 bits per second; the Bell 208 operating at 4800 bits per second; and the Bell Touch Tone which operates at a set of different frequencies. At the computer interface the modems are handled with similar protocols. However, at the telephone line interface the modems operate with distinct protocols. A terminal user communicating with a computer must communicate with a modem at the computer that is compatible with the modem utilized by the terminal. Thus, even though the computer port is capable of satisfying a wide range of modems, in order to service terminals having differing types of modems a separate computer port must be dedicated to each modem type serviced. This is economically wasteful because the use of any one particular modem is limited to a total of five to ten minutes per day. This means that each computer port is inactive a large portion of the time.
The existence of more than one modem type obligates a time sharing company to dedicate ports and telephone lines to data rate subsets corresponding to modem speeds and not to the computer port speed. Dividing ports into various speeds generally reduces system efficiency and bandwidth utilization. More telephone lines must be used to minimize customer busy conditions than would be needed if only one modem type existed. The more telephone lines needed means more adapter hardware at the computer ports and usually additional concentrator common equipment. Separate modem groups also require separate telephone numbers creating service confusion and additional costs.